Nucleotide sequence of the gene for the hole-forming toxin aerolysin of Aeromonas hydrophila.

The gene for the hole-forming toxin aerolysin from Aeromonas hydrophila was sequenced. Although most of the sequence seems unrelated to that of Staphylococcus aureus alpha-toxin, both proteins are very hydrophilic, and they each contain a nearly identical string of 10 amino acids.     

Kinetic characterization of yeast alcohol dehydrogenases. Amino acid residue 294 and substrate specificity

A three-dimensional model of yeast alcohol dehydrogenase, based on the homologous horse liver enzyme, was used to compare the substrate binding pockets of the three isozymes (I, II, and III) from Saccharomyces cerevisiae and the enzyme from Schizosaccharomyces pombe. Isozyme I and the S. pombe enzyme have methionine at position 294 (numbered as in the liver enzyme, corresponding to 270 in yeast), whereas isozymes II and III have leucine. Otherwise the active sites of the S. cerevisiae enzymes are the same. All four wild-type enzymes were produced from the cloned genes. In addition, oligonucleotide-directed mutagenesis was used to change Met-294 in alcohol dehydrogenase I to leucine. The mechanisms for all five enzymes were predominantly ordered with ethanol (but partially random with butanol) at pH 7.3 and 30 degrees C. The wild-type alcohol dehydrogenases and the leucine mutant had similar kinetic constants, except that isozyme II had 10-20-fold smaller Michaelis and inhibition constants for ethanol. Thus, residue 294 is not responsible for this difference. Apparently, substitutions outside of the substrate binding pocket indirectly affect the interactions of the alcohol dehydrogenases with ethanol. Nevertheless, the substitution of methionine with leucine in the substrate binding site of alcohol dehydrogenase I produced a 7-10-fold increase in reactivity (V/Km) with butanol, pentanol, and hexanol. The higher activity is due to tighter binding of the longer chain alcohols and to more rapid hydrogen transfer.     

ERp99, an abundant, conserved glycoprotein of the endoplasmic reticulum, is homologous to the 90-kDa heat shock protein (hsp90) and the 94-kDa glucose regulated protein (GRP94)

We have isolated an expressible full-length cDNA clone encoding murine ERp99, an abundant, conserved transmembrane glycoprotein of the endoplasmic reticulum membrane. ERp99 is synthesized as a 92,475-kDa precursor containing 802 amino acids. It possesses a signal peptide of 21 amino acids which is cleaved cotranslationally. Analysis of the amino acid sequence deduced from the nucleotide sequence of the cDNA clone led us to propose a model for the orientation of ERp99 in the endoplasmic reticulum membrane. In this model, ERp99 possesses one membrane-spanning, stop transfer segment in the N-terminal region. The protein chain passes through the membrane only once, and approximately 75% of the protein remains on the cytoplasmic side of the ER membrane. Comparison of the ERp99 sequence to the sequence of other proteins revealed that ERp99 has extensive homology with the 90-kDa heat shock protein of Saccharomyces cerevisiae (hsp90) and the 83-kDa heat shock protein of Drosophila melanogaster. In addition, the N terminus of mature ERp99 is identical to that of the 94-kDa glucose regulated protein (GRP94) of mammalian cells.     

Kinetics of intracellular transport and sorting of lysosomal membrane and plasma membrane proteins

We have used monospecific antisera to two lysosomal membrane glycoproteins, lgp120 and a similar protein, lgp110, to compare the biosynthesis and intracellular transport of lysosomal membrane components, plasma membrane proteins, and lysosomal enzymes. In J774 cells and NRK cells, newly synthesized lysosomal membrane and plasma membrane proteins (the IgG1/IgG2b Fc receptor or influenza virus hemagglutinin) were transported through the Golgi apparatus (defined by acquisition of resistance to endo-beta-N-acetylglucosaminidase H) with the same kinetics (t1/2 = 11-14 min). In addition, immunoelectron microscopy of normal rat kidney cells showed that lgp120 and vesicular stomatitis virus G-protein were present in the same Golgi cisternae demonstrating that lysosomal and plasma membrane proteins were not sorted either before or during transport through the Golgi apparatus. To define the site at which sorting occurred, we compared the kinetics of transport of lysosomal and plasma membrane proteins and a lysosomal enzyme to their respective destinations. Newly synthesized proteins were detected in dense lysosomes (lgp's and beta-glucuronidase) or on the cell surface (Fc receptor or hemagglutinin) after the same lag period (20-25 min), and accumulated at their final destinations with similar kinetics (t1/2 = 30-45 min), suggesting that these two lgp's are not transported to the plasma membrane before reaching lysosomes. This was further supported by measurements of the transport of membrane-bound endocytic markers from the cell surface to lysosomes, which exhibited additional lag periods of 5-15 min and half-times of 1.5-2 h. The time required for transport of newly synthesized plasma membrane proteins to the cell surface, and for the transport of plasma membrane markers from the cell surface to lysosomes would appear too long to account for the rapid transport of lgp's from the Golgi apparatus to lysosomes. Thus, the observed kinetics suggest that lysosomal membrane proteins are sorted from plasma membrane proteins at a post-Golgi intracellular site, possibly the trans Golgi network, before their delivery to lysosomes.     

Batrachotoxin-modified sodium channels in planar lipid bilayers. Characterization of saxitoxin- and tetrodotoxin-induced channel closures

The guanidinium toxin-induced inhibition of the current through voltage-dependent sodium channels was examined for batrachotoxin-modified channels incorporated into planar lipid bilayers that carry no net charge. To ascertain whether a net negative charge exists in the vicinity of the toxin-binding site, we studied the channel closures induced by tetrodotoxin (TTX) and saxitoxin (STX) over a wide range of [Na+]. These toxins carry charges of +1 and +2, respectively. The frequency and duration of the toxin-induced closures are voltage dependent. The voltage dependence was similar for STX and TTX, independent of [Na+], which indicates that the binding site is located superficially at the extracellular surface of the sodium channel. The toxin dissociation constant, KD, and the rate constant for the toxin-induced closures, kc, varied as a function of [Na+]. The Na+ dependence was larger for STX than for TTX. Similarly, the addition of tetraethylammonium (TEA+) or Zn++ increased KD and decreased kc more for STX than for TTX. These differential effects are interpreted to arise from changes in the electrostatic potential near the toxin-binding site. The charges giving rise to this potential must reside on the channel since the bilayers had no net charge. The Na+ dependence of the ratios KDSTX/KDTTX and kcSTX/kcTTX was used to estimate an apparent charge density near the toxin-binding site of about -0.33 e X nm-2. Zn++ causes a voltage-dependent block of the single-channel current, as if Zn++ bound at a site within the permeation path, thereby blocking Na+ movement. There was no measurable interaction between Zn++ at its blocking site and STX or TTX at their binding site, which suggests that the toxin-binding site is separate from the channel entrance. The separation between the toxin-binding site and the Zn++ blocking site was estimated to be at least 1.5 nm. A model for toxin-induced channel closures is proposed, based on conformational changes in the channel subsequent to toxin binding.     

Cellular subsets involved in cell-mediated immunity to murine Plasmodium yoelii 17X malaria

Cell mediated immunity to nonlethal Plasmodium yoelli 17X (PY17X-NL) was examined in the CBA/CaJ mouse by adoptive transfer of sensitized T lymphocyte subsets. In intact mice, PY17X-NL causes a self-limiting infection with parasitemia levels ranging from 10 to 25% of total red blood cells. Upon recovery, mice are refractory to subsequent challenge with the homologous parasite. In T cell-depleted mice, PY17X-NL infections are extremely virulent and result in death of the host after parasitemia levels reach 50% or higher. The transfer of either Lyt-1 T cells or Lyt-2 T cells from immune animals into normal, naive animals produced accelerated recovery to subsequent infection. However, this adoptive transfer of immunity by either subset was dependent upon the presence of an I-J+, Lyt-null cell in the immune population. T cell deprivation precluded the ability of animals to control blood-stage infections. When T cell-depleted mice were reconstituted with naive, Ig-negative (T cell-enriched) spleen cells, parasitemia levels were controlled and the parasites were eliminated. When T cell-deprived animals were reconstituted with naive Lyt-1+2-, Ig-negative spleen cells, they experienced twofold higher parasitemias of longer duration than mice receiving unfractionated cells. Two of six of these Lyt-1 mice died of fulminant infections, suggesting that the presence of naive Lyt-2 cells enhances the degree of protection. Immune Lyt-2 T cells were highly protective in T cell-depleted animals. Protection by sensitized Lyt-1 T cells correlated with the induction of a monocytosis. On the other hand, protection by Lyt-2T cells occurred in the absence of monocytosis. The possibility that the immunity induced by each T cell subset is mediated by a different effector mechanism is discussed.     

Induction of anti-AKR/gross virus cytolytic T lymphocytes in AKR.H-2b:Fv-1b congenic mice: age-dependent conversion to a nonresponder phenotype

Previously, we reported that the generation of cytolytic T lymphocytes (CTL) specific for syngeneic tumors induced by AKR/Gross leukemia viruses was under multi-gene control. Thus, although carrying the required immune response gene(s) encoded by the H-2b haplotype and characteristic of responder strains such as C57BL/6, AKR.H-2b congenic mice failed to mount antiviral CTL responses. Young adult AKR.H-2b:Fv-1b "doubly congenic" mice, however, were able to generate specific anti-AKR/Gross virus CTL activity. These results demonstrated that the positive effect of MHC-encoded immune response gene control could be overcome by the action of the Fv-1n allele. The responder status of the B6.Fv-1n congenic, however, indicated that this Fv-1n-mediated inhibition was dependent on the interaction of Fv-1n with another gene(s) encoded by the AKR background. The results of experiments performed with AKXL recombinant inbred mice further suggested that a single additional genetic locus, encoding the Akv-1 provirus, was necessary along with Fv-1n to cause inhibition of antiviral CTL generation. Here we show that the responsiveness of AKR.H-2b:Fv-1b mice is dependent on their age. Thus, with moderate aging these doubly congenic mice converted to a nonresponder status with respect to anti-AKR/Gross virus CTL production: 85% of mice less than or equal to 9 wk of age responded compared with 0% of mice greater than 9 wk old. As with nonresponder AKR.H-2b mice, an inverse correlation was observed between CTL responsiveness and the expression of CTL-defined viral antigens by normal cells. Namely, spleen cells from young AKR.H-2b:Fv-1b mice showed little or no expression of such viral antigens, whereas with moderate aging there was a steady increase in their display. These results are discussed with reference to possible mechanisms of unresponsiveness of AKR.H-2b vs moderately aged AKR.H-2b:Fv-1b mice, and with respect to the utility of this system as a model for naturally occurring retrovirus infections and the interactions of retroviruses with the immune system.     

Trifluoperazine Activates and Releases Latent ATP-Generating Enzymes Associated with the Synaptic Plasma Membrane

Neurone-specific enolase (NSE) and the brain form of creatine phosphokinase (CPK-BB) were previously found to be present in rat synaptosomal plasma membranes (SPM) using two-dimensional gel (2-D gel) and peptide analysis; enzymatic activities of these and of pyruvate kinase (PK), all involved in ATP generation, were shown to be "cryptic" unless the SPM were treated with Triton X-100. We now show that enzymatic activation also occurs when the SPM are treated with trifluoperazine (TFP). TFP activation occurred even when the enzymes were membrane associated, showing that solubilization was not responsible for "unmasking" the enzyme activities. When TFP treatment was performed at alkaline instead of neutral pH, NSE and CPK-BB were released as well as PK, nonneuronal enolase, and aldolase which were identified by 2-D gel and tryptic peptide analysis. Other proteins released included calmodulin, actin, and the 70-kilodalton heat-shock cognate protein. Tubulin, synapsin I, and a 35-kilodalton basic protein were largely unaffected. The latter was identified as the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase on the basis of 2-D gel and peptide analyses and subsequent partial sequencing of a rat brain cDNA coding for the same protein. TFP treatment is thus useful for activating latent enzymes as well as for distinguishing enzymes that have a different disposition on the membrane.     

Linkage studies with chromosome 17 DNA markers in 45 neurofibromatosis 1 families

A locus for von Recklinghausen neurofibromatosis (NF1) has recently been mapped near the chromosome 17 centromere. We have extended these linkage studies by genotyping 45 NF1 families with three DNA probes known to be linked to the chromosome 17 centromeric region. Of 34 families informative for NF1 and at least one of the three probes, 28 families show no recombinants with the disease gene. These data provide additional support for genetic homogeneity of NF1 and for a primary NF1 locus linked to the chromosome 17 centromere. Among the informative families were 7 families with apparent new NF1 mutations. Our data suggest that these mutations are probably at the chromosome 17 NF1 locus.     

Skeletal muscle sarcolemma in malignant hyperthermia: evidence for a defect in calcium regulation

Sarcolemmal properties implicated in the skeletal muscle disorder, malignant hyperthermia (MH), were examined using sarcolemma-membrane vesicles isolated from normal and MH-susceptible (MHS) porcine skeletal muscle. MHS and normal sarcolemma did not differ in the distribution of the major proteins, cholesterol or phospholipid content, vesicle size and sidedness, (Na+ + K+)-ATPase activity, ouabain binding, or adenylate cyclase activity (total and isoproterenol sensitivity). The regulation of the initial rates of MHS and normal sarcolemmal ATP-dependent calcium transport (calcium uptake after 1 min) by Ca2+ (K1/2 = 0.64-0.81 microM), calmodulin, and cAMP-dependent protein kinase were similar. However, when sarcolemmal calcium content was measured at either 2 or 20 min after the initiation of active calcium transport, a significant difference between MHS and normal sarcolemmal calcium uptake became apparent, with MHS sarcolemma accumulating approximately 25% less calcium than normal sarcolemma. Calcium transport by MHS and normal sarcolemma, at 2 or 20 min, had a similar calmodulin dependence (C1/2 = 150 nM), and was stimulated to a similar extent by cAMP-dependent protein kinase or calmodulin. Halothane inhibited MHS and normal sarcolemmal active calcium uptake in a similar fashion (half-maximal inhibition at 10 mM halothane), while dantrolene (30 microM) and nitrendipine (1 microM) had little effect on either MHS or normal sarcolemmal calcium transport. After 20 min of ATP-supported calcium uptake, 2 mM EGTA plus 10 microM sodium orthovanadate were added to initiate sarcolemmal calcium efflux. Following an initial rapid phase of calcium release, an extended slow phase of calcium efflux (k = 0.012 min-1) was similar for both MHS and normal sarcolemma vesicles. We conclude that although a number of sarcolemmal properties, including passive calcium permeability, are normal in MH, a small but significant defect in MHS sarcolemmal ATP-dependent calcium transport may contribute to the abnormal calcium homeostasis and altered contractile properties of MHS skeletal muscle.